The present invention relates to radial piston fuel supply pumps, and particularly to single piston pumps for pressurizing common rail fuel injection systems.
Single piston, cam driven high pressure pumps have become a common solution for generating high pressure fuel in common rail, direct injection, gasoline engines. These pumps are typically driven by a tappet mounted adjacent to a valve cam for cyclically pushing on the actuated end of the pumping piston. In the case of overhead cam engine applications, a short, light weight tappet is used and the overall reciprocating mass of the pump system is manageable with a single return spring mounted at the exterior of the fuel pump. This spring directly returns the piston and the piston simultaneously returns the tappet. However, when adapting direct injection technology to a conventional push-rod type V-6 or V-8 engine with a single cam shaft, it becomes evident that longer, heavier tappets must be managed. In this case the cam shaft is centrally located in the engine, and the desired position of the pump is atop the engine, to accommodate fuel connection access. The added reach results in a longer tappet arrangement and increased reciprocating mass. This significant increase in mass requires return spring loads that can be more than two times the typical loads in overhead cam engines.
The conventional piston return spring is located between the pump body and a spring seat mounted on the actuated end of the piston. Such return springs provide the dual functions of returning the plunger and returning the tappet. Increasing the size of a single return spring presents two problems. First, trying to package a longer, more powerful spring while maintaining the same extension of the piston outside the pump body, becomes difficult and very costly. Second, a more powerful spring can impart significant unwanted side loads on the pumping piston, which can produce piston seizures. The uneven loads are caused by normal spring end squareness tolerances, and eccentric loading (offset from centerline) caused by spring geometry variations.